Question: Which of these numbers is composite? ${21,\ 43,\ 53,\ 61,\ 73}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 43, 53, 61, and 73 each have only two factors. The factors of 21 are 1, 3, 7, and 21. Thus, 21 is the composite number.